NAME

ovdb - Overview storage method for INN

DESCRIPTION

Ovdb is a storage method that uses the Berkeley DB library to store
overview data. It requires version 4.4 or later of the Berkeley DB
library (4.7+ is recommended because older versions suffer from various
issues).
Ovdb makes use of the full transaction/logging/locking functionality of
the Berkeley DB environment. Berkeley DB may be downloaded from
<http://www.sleepycat.com> and is needed to build the ovdb backend.

UPGRADING

This is version 2 of ovdb. If you have a database created with a
previous version of ovdb (such as the one shipped with INN 2.3.0) your
database will need to be upgraded using ovdb_init(8). See the man page
ovdb_init(8) for upgrade instructions.

INSTALLATION

To build ovdb support into INN, specify the option --with-berkeleydb
when running the configure script. By default, configure will search
for a Berkeley DB tree in several likely locations, and choose the
highest version (based on the name of the directory, e.g.,
BerkeleyDB.4.4) that it finds. There will be a message in the
configure output indicating the chosen pathname.
You can override this pathname by adding a path to the option, e.g.,
--with-berkeleydb=/usr/BerkeleyDB.4.4. This directory is expected to
have subdirectories include and lib, containing db.h, and the library
itself, respectively.
The ovdb database may take up more disk space for a given spool than
the other overview methods. Plan on needing at least 1.1 KB for every
article in your spool (not counting crossposts). So, if you have 5
million articles, you’ll need at least 5.5 GB of disk space for ovdb.
With compression enabled, this estimate changes to 0.7 KB per article.
See the COMPRESSION section below. Plus, you’ll need additional space
for transaction logs: at least 100 MB. By default the transaction logs
go in the same directory as the database. To improve performance, they
can be placed on a different disk -- see the DB_CONFIG section.

CONFIGURATION

To enable ovdb, set the ovmethod parameter in inn.conf to "ovdb". The
ovdb database is stored in the directory specified by the pathoverview
paramter in inn.conf. This is the "DB_HOME" directory. To start out,
this directory should be empty (other than an optional DB_CONFIG file;
see DB_CONFIG for details) and innd (or makehistory) will create the
files as necessary in that directory. Make sure the directory is owned
by the news user.
Other parameters for configuring ovdb are in the ovdb.conf(5)
configuration file. See also the sample ovdb.conf.
cachesize
Size of the memory pool cache, in kilobytes. The cache will have a
backing store file in the DB directory which will be at least as
big. In general, the bigger the cache, the better. Use "ovdb_stat
-m" to see cache hit percentages. To make a change of this
parameter take effect, shut down and restart INN (be sure to kill
all of the nnrpds when shutting down). Default is 8000, which is
adequate for small to medium sized servers. Large servers will
probably need at least 20000.
compress
If INN was compiled with zlib, and this compress parameter is true,
OVDB will compress overview records that are longer than 600 bytes.
See the COMPRESSION section below.
numdbfiles
Overview data is split between this many files. Currently, innd
will keep all of the files open, so don’t set this too high or innd
may run out of file descriptors. nnrpd only opens one at a time,
regardless. May be set to one, or just a few, but only do that if
your OS supports large (>2G) files. Changing this parameter has no
effect on an already-established database. Default is 32.
txn_nosync
If txn_nosync is set to false, Berkeley DB flushes the log after
every transaction. This minimizes the number of transactions that
may be lost in the event of a crash, but results in significantly
degraded performance. Default is true.
useshm
If useshm is set to true, Berkeley DB will use shared memory
instead of mmap for its environment regions (cache, lock, etc).
With some platforms, this may improve performance. Default is
false.
shmkey
Sets the shared memory key used by Berkeley DB when ’useshm’ is
true. Berkeley DB will create several (usually 5) shared memory
segments, using sequentially numbered keys starting with ’shmkey’.
Choose a key that does not conflict with any existing shared memory
segments on your system. Default is 6400.
pagesize
Sets the page size for the DB files (in bytes). Must be a power of
2. Best choices are 4096 or 8192. The default is 8192. Changing
this parameter has no effect on an already-established database.
minkey
Sets the minimum number of keys per page. See the Berkeley DB
documentation for more info. Default is based on page size and
whether compression is enabled:
default_minkey = MAX(2, pagesize / 2600) if compress is false
default_minkey = MAX(2, pagesize / 1500) if compress is true
The lowest allowed minkey is 2. Setting minkey higher than the
default is not recommended, as it will cause the databases to have
a lot of overflow pages. Changing this parameter has no effect on
an already-established database.
maxlocks
Sets the Berkeley DB "lk_max" parameter, which is the maximum
number of locks that can exist in the database at the same time.
Default is 4000.
nocompact
The nocompact parameter affects expireover’s behavior. The
expireover function in ovdb can do its job in one of two ways: by
simply deleting expired records from the database, or by re-writing
the overview records into a different location leaving out the
expired records. The first method is faster, but it leaves ’holes’
that result in space that can not immediately be reused. The
second method ’compacts’ the records by rewriting them.
If this parameter is set to 0, expireover will compact all
newsgroups; if set to 1, expireover will not compact any
newsgroups; and if set to a value greater than one, expireover will
only compact groups that have less than that number of articles.
Experience has shown that compacting has minimal effect (other than
making expireover take longer) so the default is now 1. This
parameter will probably be removed in the future.
readserver
Normally, each nnrpd process directly accesses the Berkeley DB
environment. The process of attaching to the database (and
detaching when finished) is fairly expensive, and can result in
high loads in situations when there are lots of reader connections
of relatively short duration.
When the readserver parameter is true, the nnrpds will access
overview via a helper server (ovdb_server -- which is started by
ovdb_init). This can also result in cleaner shutdowns for the
database, improving stability and avoiding deadlocks and corrupted
databases. If you are experiencing any instability in ovdb, try
setting this parameter to true. Default is false.
numrsprocs
This parameter is only used when readserver is true. It sets the
number of ovdb_server processes. As each ovdb_server can process
only one transaction at a time, running more servers can improve
reader response times. Default is 5.
maxrsconn
This parameter is only used when readserver is true. It sets a
maximum number of readers that a given ovdb_server process will
serve at one time. This means the maximum number of readers for
all of the ovdb_server processes is (numrsprocs * maxrsconn). This
does not limit the actual number of readers, since nnrpd will fall
back to opening the database directly if it can’t connect to a
readserver. Default is 0, which means an umlimited number of
connections is allowed.

COMPRESSION

New in this version of OVDB is the ability to compress overview data
before it is stored into the database. In addition to consuming less
disk space, compression keeps the average size of the database keys
smaller. This in turn increases the average number of keys per page,
which can significantly improve performance and also helps keep the
database more compact. This feature requires that INN be built with
zlib. Only records larger than 600 bytes get compressed, because that
is the point at which compression starts to become significant.
If compression is not enabled (either from the "compress" option in
ovdb.conf or INN was not built from zlib), the database will be
backward compatible with older versions of OVDB. However, if
compression is enabled, the database is marked with a newer version
that will prevent older versions of OVDB from opening the database.
You can upgrade an existing database to use compression simply by
setting compress to true in ovdb.conf. Note that existing records in
the database will remain uncompressed; only new records added after
enabling compression will be compressed.
If you disable compression on a database that previously had it
enabled, new records will be stored uncompressed, but the database will
still be incompatible with older versions of OVDB (and will also be
incompatible with this version of OVDB if it was not built with zlib).
So to downgrade to a completely uncompressed database you will have to
rebuild the database using makehistory.

DB_CONFIG

A file called DB_CONFIG may be placed in the database directory to
customize where the various database files and transaction logs are
written. By default, all of the files are written in the "DB_HOME"
directory. One way to improve performance is to put the transaction
logs on a different disk. To do this, put:
DB_LOG_DIR /path/to/logs
in the DB_CONFIG file. If the pathname you give starts with a /, it is
treated as an absolute path; otherwise, it is relative to the "DB_HOME"
directory. Make sure that any directories you specify exist and have
proper ownership/mode before starting INN, because they won’t be
created automatically. Also, don’t change the DB_CONFIG file while
anything that uses ovdb is running.
Another thing that you can do with this file is to split the overview
database across multiple disks. In the DB_CONFIG file, you can list
directories that Berkeley DB will search when it goes to open a
database.
For example, let’s say that you have pathoverview set to /mnt/overview
and you have four additional file systems created on /mnt/ov?. You
would create a file "/mnt/overview/DB_CONFIG" containing the following
lines:
set_data_dir /mnt/overview
set_data_dir /mnt/ov1
set_data_dir /mnt/ov2
set_data_dir /mnt/ov3
set_data_dir /mnt/ov4
Distribute your ovNNNNN files into the four filesystems. (say, 8
each). When called upon to open a database file, the db library will
look for it in each of the specified directories (in order). If said
file is not found, one will be created in the first of those
directories.
Whenever you change DB_CONFIG or move database files around, make sure
all news processes that use the database are shut down first (including
nnrpds).
The DB_CONFIG functionality is part of Berkeley DB itself, rather than
something provided by ovdb. See the Berkeley DB documentation for
complete details for the version of Berkeley DB that you’re running.

RUNNING

When starting the news system, rc.news will invoke ovdb_init.
ovdb_init must be run before using the database. It performs the
following tasks:
· Creates the database environment, if necessary.
· If the database is idle, it performs a normal recovery. The
recovery will remove stale locks, recreate the memory pool cache,
and repair any damage caused by a system crash or improper
shutdown.
· Starts the DB housekeeping processes (ovdb_monitor) if they’re not
already running.
And when stopping INN, rc.news kills the ovdb_monitor processes after
the other INN processes have been shut down.

DIAGNOSTICS

Problems relating to ovdb are logged to news.err with "OVDB" in the
error message.
INN programs that use overview will fail to start up if the
ovdb_monitor processes aren’t running. Be sure to run ovdb_init before
running anything that accesses overview.
Also, INN programs that use overview will fail to start up if the user
running them is not the "news" user.
If a program accessing the database crashes, or otherwise exits
uncleanly, it might leave a stale lock in the database. This lock
could cause other processes to deadlock on that stale lock. To fix
this, shut down all news processes (using "kill -9" if necessary) and
then restart. ovdb_init should perform a recovery operation which will
remove the locks and repair damage caused by killing the deadlocked
processes.

FILES

inn.conf
The ovmethod and pathoverview parameters are relevant to ovdb.
ovdb.conf
Optional configuration file for tuning. See CONFIGURATION above.
pathoverview
Directory where the database goes. Berkeley DB calls it the
’DB_HOME’ directory.
pathoverview/DB_CONFIG
Optional file to configure the layout of the database files.
pathrun/ovdb.sem
A file that gets locked by every process that is accessing the
database. This is used by ovdb_init to determine whether the
database is active or quiescent.
pathrun/ovdb_monitor.pid
Contains the process ID of ovdb_monitor.

TODO

Implement a way to limit how many databases can be open at once (to
reduce file descriptor usage); maybe using something similar to the
cache code in ov3.c